1. Field of the Invention
The present invention relates to methods and apparatuses for fetching reference information. More specifically, the present invention relates to methods and apparatuses for fetching, at high speeds, reference information that is classified and stored in a plurality of storage locations according to predetermined priorities.
2. Description of the Prior Art
In a logic simulator, which is one of the design tools for LSI, a plurality of pieces of cell information is stored in a file format in disk storage, each of which describes a logic model of each cell that is a basic element for various circuits, and a simulation is executed while fetching required pieces among the cell information from the disk storage. Each file, which is a unit for reference information and represents each cell, is classified and stored in one of plural data storage locations (directories) in the disk storage. Since a cell information group, which comprises plural pieces of cell information, is usually stored in one directory, plural pieces of cell information that have an identical file name do not exist in more than two directories. However, for example, when a cell information group that is developed for a specific user is managed in a separate directory, or when a part of cell information is changed with regard to internal property data and is then copied to another directory with a same name, plural files of the same name will overlap and exist in two or more different directories.
FIG. 11 shows such an example as described above. There are three directories AA, BB, and CC in disk storage, and plural pieces of cell information, such as AND3, NOT, etc., are stored separately in a file format in these directories AA, BB, and CC. Hereinafter, AND3, OR2, etc., are fetched as cell names. The cell information whose name is OR2, OR3, AND3, or NOT overlaps and exists in plural directories. In this case, for the cell information that simultaneously exists in plural directories with the same cell name, it is necessary beforehand to determine a priority order for each directory that has the same cell information to be fetched in order to prevent duplicate fetching of identical cell information.
For example, in the above-mentioned example, the priority order for the directories is determined as AA&gt;BB&gt;CC, and each cell information should be fetched according to the following access conditions.
Although OR2 exists in AA and BB, OR2 of AA should be accessed. PA0 Although OR3 exists in BB and CC, OR3 of BB should be accessed. PA0 Although AND3 exists in AA and CC, AND3 of AA should be accessed. PA0 Although NOT exists in AA, BB, and CC, NOT of AA should be accessed. PA0 Since OR4 exists only in AA, it should be accessed. PA0 Since AND2 exists only in BB, it should be accessed. PA0 Since AND4 exists only in CC, it should be accessed.
According to conventional methods, a priority table as shown in FIG. 12 should be prepared beforehand in order to realize such access. As shown in FIG. 13, whenever there is an access request (S41), each directory is searched one by one, based on this table (S42-S44), and when the cell information of a target cell name is found, the retrieval of the cell information is terminated (S45-S48).
However, according to such a method, because it is necessary to search, in order, for two or more information storage locations (directories), when there are many cases to refer to and many storage locations, there will be a problem in that the process takes considerable time.
This invention was made in view of such a problem, and it is an object of the present invention to provide methods and apparatuses for fetching information that is classified and stored, which can shorten the time required when a series of fetching operations is carried out.